Emulsion adhesive for washable film

ABSTRACT

This invention relates to removable pressure sensitive adhesive compositions and constructions that contain an emulsion acrylic pressure sensitive adhesive copolymer of a plurality of acrylic monomers and at least one polymerizable acid. The invention further relates to labels, particularly removable polymeric labels for use on reusable containers.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application U.S. Ser.No. 60/864,028 filed Nov. 2, 2006. The provisional application is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to acrylic emulsion pressure sensitive adhesives,and more particularly, to acrylic adhesives for use in removablepolymeric film labels.

BACKGROUND

It is common practice to apply labels to containers or bottles formedfrom polymers or glass. Such containers and bottles are available in awide variety of shapes and sizes for holding many different types ofmaterials such as detergents, chemicals, motor oil, beverages, includingjuices, soft drinks, alcoholic beverages, etc. The labels provideinformation such as the supplier of the container or the contents of thecontainer.

Polymeric film materials and film facestocks have been described for useas labels in various fields. Polymeric labels are increasingly desiredfor many applications, particularly clear polymeric labels since theyprovide a no-label look to decorated glass and plastic containers. Paperlabels block the visibility of the container and/or the contents in thecontainer. Clear polymeric labels enhance the visual aesthetics of thecontainer, and therefore the product, and are growing much faster thanpaper labels in the package decoration market as consumer productcompanies are continuously trying to upgrade the appearance of theirproducts. Polymeric film labels also have superior mechanicalproperties, such as tensile strength and abrasion resistance.

In the bottled beverage industry, particularly the bottled beerindustry, the standards to which the labels are held can be quitestringent. For example, the labeled bottles must withstand thepasteurization process. The labels must be abrasion resistant because ofthe demands of the bottling, packing, shipping and storage processes.The labels must also survive being immersed in ice water for extendedperiods of time.

In addition, the bottles used in the beverage industry are generallyreused many times. The bottles must be cleaned and the labels removedprior to refilling and relabeling the bottles. Paper labels, while beinggenerally less aesthetically desirable, are easily removed during thewashing process in which the bottles are subjected to hot washing liquidsuch as dilute caustic soda that has been heated to 50-90° C. Becausepolymeric labels do not possess the water permeability of the paperlabels, the polymeric labels have been found to be more difficult tocompletely remove with the existing washing process.

Accordingly, it would be desirable to produce polymeric film labels thatcan be completely removed from the bottles during the washing process,yet maintain their superior aesthetic and mechanical properties. Itwould also be desirable to produce a polymeric label having a pressuresensitive adhesive that is capable of being coated onto the polymericfilm at high speeds and that can be quickly and cleanly removed from thebottle or substrate.

SUMMARY

The present invention provides an acrylic emulsion pressure sensitiveadhesive (PSA) that removes quickly and cleanly from glass andpolymer-coated glass bottles in water at elevated temperatures anddilute caustic solution.

An emulsion based pressure sensitive adhesive composition is providedthat comprises a pressure sensitive adhesive copolymer of a plurality ofcopolymerizable monomers comprising at least one polymerizable acid,wherein the composition is neutralized to a pH of greater than 4.5 withat least one hydroxy functional amine, and wherein the composition isremovable upon exposure to an aqueous caustic solution at a temperatureof at least 50° C.

In one embodiment, the adhesive composition includes a pressuresensitive adhesive copolymer comprising a copolymer of (a) about 75% toabout 90% by weight of one or more alkyl acrylates having from about 4to about 12 carbon atoms in the alkyl group; (b) about 0.25% to about 5%by weight of one or more acid monomers; (c) about 10% to about 20% byweight of at least one hard monomer; (d) 0 to about 1% by weight of atleast one multifunctional crosslinking monomers.

The pressure sensitive adhesive is particularly useful for a removablelabel for reusable containers. In one embodiment, there is provided aremovable label comprising. (a) a first polymeric layer having a firstcoefficient of thermal expansion; (b) a second polymeric layer having asecond coefficient of linear thermal expansion underlying the firstpolymeric layer, wherein the first coefficient of linear thermalexpansion is less than the second coefficient of linear thermalexpansion; and (c) an adhesive layer underlying the second polymericlayer, the adhesive layer comprising an emulsion based pressuresensitive adhesive composition comprising a pressure sensitive adhesivecopolymer of a plurality of copolymerizable monomers comprising at leastone polymerizable acid; wherein the composition is neutralized to a pHof greater than 4.5 with at least one hydroxy functional amine; andwherein the label is reversibly curled toward the first polymeric layerat a temperature at or above 50° C. and the adhesive is removable uponexposure to an aqueous caustic solution at a temperature of at least 50°C.

In another embodiment, there is provided a removable label comprising.(a) a polymeric layer comprising at least one shrinkable film whereinthe film is stretched in at least one direction; (b) an adhesive layerunderlying the polymeric layer, the adhesive layer comprising anemulsion based pressure sensitive adhesive composition comprising apressure sensitive adhesive copolymer of a plurality of copolymerizablemonomers comprising at least one polymerizable acid; wherein thecomposition is neutralized to a pH of greater than 4.5 with at least onehydroxy functional amine; and wherein the polymeric layer irreversiblyshrinks at a temperature at or above 50° C. and the adhesive isremovable upon exposure to an aqueous caustic solution at a temperatureof at least 50° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross section of a label construction of the presentinvention.

FIG. 1B is a cross section of the label of FIG. 1A as applied to acylindrical container.

FIGS. 2-9 are cross sections of embodiments of the label constructionsof the present invention.

DETAILED DESCRIPTION

In one embodiment of the present invention, the acrylic emulsionpressure sensitive adhesive contains a pressure sensitive adhesivecopolymer of a plurality of copolymerizable monomers. Thecopolymerizable monomers include (a) one or more alkyl acrylates havingfrom about 4 to about 12 carbon atoms in the alkyl group; (b) one ormore acid monomers; (c) at least one hard monomer; and (d) optionally,at least one multifunctional crosslinking monomers.

The acrylic emulsion pressure sensitive adhesive copolymer, in oneembodiment, comprises a plurality of copolymerizable monomers, the majorportion of which is one or more alkyl acrylates having about 4 to about12 carbon atoms in the alkyl group. Non-limiting examples of alkylacrylates having about 4 to about 12 carbon atoms in the alkyl groupinclude butyl acrylate, hexyl acrylate, heptyl acrylate, n-octylacrylate, iso-octyl acrylate, 2-ethylhexyl acrylate, isobutyl acrylateand nonyl acrylate. In one embodiment, the copolymer comprises about 75%to about 90% by weight of the alkyl acrylate having from about 4 toabout 12 carbon atoms in the alkyl group. In another embodiment, thecopolymer comprises about 77% to about 85% by weight of the alkylacrylate.

The copolymer comprises about 0.25% up to about 5% by weight, or fromabout 0.5% to about 3% by weight of at least one unsaturated carboxylicacid containing from 3 to about 8 carbon atoms. Non-limiting examples ofethylenically unsaturated carboxylic acids include acrylic acid,methacrylic acid, beta-carboxyethyl acrylate, itaconic acid andbeta-carboxyethyl acrylate.

As used herein, the term “multifunctional crosslinking monomer” means amonomer means a monomer that is copolymerizable with acrylic monomersand that has at least one carbon-carbon double bond and at least oneother functional group capable of entering into a polymerization orcrosslinking reaction. In one embodiment, the copolymer comprises up toabout 1% by weight of at least one multifunctional crosslinking monomer.In another embodiment, the copolymer comprises up to about 0.5%, or fromabout 0.01% to about 0.5% by weight of at least one multifunctionalcrosslinking monomer. Non-limiting examples of such monomers includemultifunctional (meth)acrylates, e.g., diacrylates, triacrylates,dimethyacrylates, and trimethylacrylates; multifunctional allyliccompounds, e.g., diallyl maleate and allyl methacrylate; multifunctionalcrosslinking monomers having a vinyl group; and multifunctionalcrosslinking monomers having a reactive heteroatom-containing functionalgroup such as the imidazolidone group. A non-limiting list includes thefollowing:

TABLE 1 Multifunctional Monomer Abbreviation Allyl Methacrylate AMADiallyl Maleate DAM Divinyl Benzene DVB Ethylene Glycol DimethacrylateEGDMA N,N′-methylene-bis-acrylamide NNMBA Diallyl Phthalate DAPButanediol Diacrylate BDD Hexanediol Diacrylate HDDA Tripropylene GlycolDiacrylate TPGDA Triallyl Cyanurate TAC Tetraethylene Glycol DiacrylateTEGDA Butanediol Dimethacrylate BDDMA Tetraethylene GlycolDimethacrylate TEDMA, TEGMA Trimethylolpropane Triacrylate TMPTATrimethylolpropane Trimethacrylate TMPTMA, TRIM TrimethylolpropaneDiallyl Ether TMPDAE Imidazolidone Methacrylate MEIO

As used herein, the term “hard monomer” refers to a monomer thehomopolymer of which is not inherently tacky at ambient temperatures andwhich has a Tg greater than about −25° C. Non-limiting examples includemethyl and ethyl acrylate, methyl and ethyl methacrylate, vinyl acetate,styrene, and acrylonitrile. In one embodiment, the copolymer comprisesabout 10% to about 20% by weight of hard monomer, or about 15% to about20% by weight of hard monomer.

The monomers may be polymerized in the presence of a surfactant or amixture of surfactants. The total amount of surfactant used can be inthe range of about 0 to about 7%, or about 1.3% to about 5% based on theweight of the polymer. Anionic surfactants, nonionic surfactants, andcombinations thereof may be used. Suitable anionic surfactants include,for example, alkyl sulfates, alkyl phenol ethoxysulfates, alkylsulfonates, alkaryl sulfonates and alkyl sulfosuccinates, such as forexample, sodium lauryl sulfate, nonyl phenol ethoxy(4)sulfate, sodiumdodecyl benzene sulfonate, disodium dioctyl sulfosuccinates, as well asmixtures thereof. Suitable nonionic surfactants include, for example,ethoxylated alkyl phenols, poly(ethylene oxide/propylene oxide) blockcopolymers, such as, for example, ethoxylated (10-60 moles ethyleneoxide) nonyl phenol, as well as mixtures thereof.

In one embodiment, a combination of surfactants is used in the course ofpolymerizing the monomers, with anionic surfactants being particularlyuseful. In particular, a mixture of two different surfactants may beused. In one embodiment, the first type of surfactant is a sulfate of anethoxylated alcohol, e.g., a sodium lauryl ether sulfate. Non-limitingexamples include Disponil FES 77 and Disponil FES 993 from Cognis Corp.and Polystep B-19 from Stepan Company. The second type of surfactant is,in one embodiment, a sulfosuccinate or derivative, e.g., a dioctyl esterof sodium sulfosuccinic acid. Non-limiting examples include AerosolOT-75 from Cytec Industries, Inc. and Disponil SUS IC 875 from CognisCorp. In another embodiment, the second type of surfactant is a modifiedfatty alcohol polyglycolether. Non-limiting examples include DisponilAFX 1080 and Disponil AFX 2075 from Cognis Corp.

In addition to the monomers and surfactants described above, additionalingredients, reagents, processing aids, and other components are used inpreparing preferred embodiments of the invention. A non-limiting listincludes polymerization catalysts (initiators), chain transfer agents,neutralizers, accelerators, tackifiers, defoaming agents, thickeners orrheology modifiers, buffers, antioxidants and biocides.

Suitable thermal polymerization initiators include, but are not limitedto, sodium persulfate, potassium persulfate, ammonium persulfate, sodiumperborated, and ammonium or alkali metal peroxy disulfate salts. Theinitiator may be used alone or in combination with a minor amount of oneor more reducing components or activators, such as, for example sodiumsulfoxylate formaldehyde, alkali metal and ammonium salts ofsulfur-containing acids, such as sodium sulfite, bisulfite, thiosulfate,hydrosulfite, sulfide, hydrosulfide or dithionite, formadinesulfinicacid, hydroxymethanesulfonic acid, acetone bisulfite, amines such asethanolamine, glycolic acid, glyoxylic acid hydrate, ascorbic acid,isoascorbic acid, lactic acid, glyceric acid, malic acid,2-hydroxy-2-sulfinatoacetic acid, tartaric acid and salts of thepreceding acids.

A chain transfer agent may be employed in the preparation of theadhesive. The weight average molecular weight (Mw) of the emulsionpolymer may be adjusted through the addition of a chain transfer agent,such as n-dodecyl mercaptan, during emulsion polymerization to give asuitable balance of adhesive and cohesive strength. Suitable chaintransfer agents include, but are not limited to, isopropanol,halogenated compounds, n-butyl mercaptan, n-amyl mercaptan, n-dodecylmercaptan, t-dodecyl mercaptan, alkyl thioglycolate, mercaptopropionicacid, and alkyl mercaptoalkanoate. According to one embodiment of theinvention, chain transfer agent is not required but in some embodimentsit is used in an amount of from 0.001 to 0.5, including from 0.0025 to0.1, moles per kg dry polymer weight. Linear or branched C₄-C₂₂ alkylmercaptans such as n-dodecyl mercaptan and t-dodecyl mercaptan areparticularly useful.

In the preparation of the acrylic emulsion polymer, a neutralizer isincluded. By “neutralizer”, what is meant is a basic material that iscapable of entering into an acid-base reaction with the acid monomer.Suitable neutralizers include poly-hydroxy functional amines, such asfor example, triethanolamine,2-[bis-(2-hydroxyethyl)-amino]-2-hydroxymethyl propane-1,3-diol (bistris methane), 2-amino-2-(hydroxymethyl)-1,3-propanediol (trizma base),N-methyl-D-glucamine, N,N,N′N′-tetrakis-(2-hydroxyethyl)-ethylenediamineand N,N,N′N′-tetrakis-(2-hydroxypropyl)-ethylenediamine.

An organic salt, for example, sodium citrate, sodium gluconate or sodiumacetate may be used in combination with the neutralizer to enhance theremovability of the adhesive. An inorganic acid salt may also be used.Alternatively, an organic acid neutralized by a hydroxy amine can beused. Non-limiting examples of organic acids include acetic acid,p-toluene sulfonic acid, citric acid, gluconic acid, maleic acid anddodecylbenzene sulfonic acid. The organic acids can be either added tothe water phase during the polymerization or added post polymerization.

Chemical crosslinkers may or may not be present in the composition. Ifpresent, the external crosslinkers are provided in an amount up to about5 by weight, based on the weight of dry adhesive, to further increasethe cohesive strength. Non-limiting examples of external crosslinkersinclude chromium acetate, zirconium ammonium carbonate, aluminumacetate, polyaziridines, carbodiimides and the like. For polyaziridines,the amount of crosslinker used is typically less than about 1% byweight, or about 0.6% or less by weight, based on the weight of dryadhesive.

The adhesive composition may contain, in addition to the emulsionacrylic copolymer, an emulsifiable wax and a plasticizer. Non-limitingexamples of emulsifiable waxes include emulsified petroleum resins,paraffin waxes, oxidized paraffin waxes, microcrystalline waxes,carnauba waxes, montan waxes, polyethylene waxes, and the like.Non-limiting examples of plasticizers include phthalates, such as butylbenzyl phthalate, and dioctyl phthalate; adipates; benzoates; citrates,such as triethyl citrate and acyl tributyl citrate; maleates; oleates;phosphates; sebacates; stearates; epoxidized vegetable oils; rosinderivatives, and polymeric plasticizers, such as low molecular weightacrylic polymers that lower the Tg and storage modulus of the PSA.

The pressure sensitive adhesives can be applied to a release liner orfacestock using conventional methods. For example, the PSA can beapplied to a release liner using slide coating, multilayer die coating,die/slide combination coating and the like, and then laminated to afacestock.

While adhesive coat weights can vary considerably, depending upon thefacestock and intended application, in general, the adhesive is appliedto a dry coat weight of from about 12 to about 25 grams per square meter(g/m²), or from 13 to about 21 g/m².

A wide variety of facestocks or backings, including paper and polymericfilm facestocks, are contemplated for use with the adhesive composition.The facestock may be primed or unprimed. The facestock may be surfacetreated by, for example, corona discharge, flame, plasma, etc, toprovide the surfaces with desirable properties such as improved adhesionto subsequently applied layers. Procedures for corona treating and flametreating of polymeric films are a well known to those skilled in theart.

The term “overlies” and cognate terms such a overlying and the like,when referring to the relationship of one or a first layer relative toanother or a second layer, refer to the fact that the first layerpartially or completely overlies the second layer. The first layeroverlying the second layer may or may not be in contact with the secondlayer. For example, one or more additional layers may be positionedbetween the first and the second layer. The term “underlies” and cognateterms such as “underlying” and the like have similar meanings exceptthat the first layer partially or completely lies under, rather thanover the second layer.

The term “transparent” when referring to one or more layers of the labelmeans any material beneath such layers can be seen through such layers.In reference to the use of the “transparent” or “clear” labels appliedto clear containers, such as beer bottles, the bottle and the beerwithin the bottle are visible through the label.

The term “clear” when referring to one or more layers of the label or tothe label itself means the opacity of the layers or label is less thanabout 5%, and the layers or the label has a haze of less than about 10%.Opacity is measured in accordance with TAPPI Test T425 os, and haze ismeasured in accordance with ASTM Test Method D-1003.

The polymeric facestock may be a monolayer film or a multilayer film.The multilayer film may comprise from two to ten or more layers. Thepolymer facestock may be oriented or not oriented. The polymer facestockmay be transparent or opaque. Opaque facestocks generally comprise apolymer as described below and one or more pigments to provide thefacestock, or one layer of a multilayer facestock with the desiredcolor. Pigments useful for this purpose are well known in the art. Forexample, white films can be prepared by introducing titanium dioxide andother white pigments into the polymer. Carbon black may be introduced toprovide a black or grey facestock or film.

A wide variety of polymer film materials are useful in preparing thepolymeric layers useful in the present invention. For example, thepolymeric film material may include polymers and copolymers such as atleast one polyolefin, polyacrylate, polystyrene, polyamide, polyvinylalcohol, poly(alkylene acrylate), poly(ethylene vinyl alcohol),poly(alkylene vinyl acetate), polyurethane, polyacrylonitrile,polyester, polyester copolymer, fluoropolymer, polysulfone,polycarbonate, styrene-maleic anhydride copolymer, styrene-acrylonitrilecopolymer, ionomers based on sodium or zinc salts of ethylenemethacrylic acid, cellulosics, polyacrylonitrile, alkylene-vinyl acetatecopolymer, or mixtures of two or more thereof.

In one embodiment, the adhesive is applied to a reversibly curlablefacestock film. Such films are described in International PublicationWO2006/076327, the disclosure of which is hereby incorporated byreference. Reversibly curlable films contain at least two polymericlayers having different coefficients of linear thermal expansion, suchthat, upon exposure to elevated temperatures, the film is reversiblycurled. These films are particularly useful for removable labels forreusable containers. The labels for reusable beverage bottles, includingbeer bottles, are subjected to hot, caustic washing liquid to remove thelabel. The temperature of the washing liquid may be greater than orequal to about 50° C., about 60° C., about 70° C., about 80° C. orhigher. As the label curls or distorts, the adhesive is exposed to thewashing liquid and the label is quickly and cleanly removed from thebottle.

As illustrated in FIG. 1A, label 10 comprises a first polymeric layer11, a second polymeric layer 12 underlying polymeric layer 11, andadhesive layer 16 underlying the second polymeric layer 12. At hightemperatures, e.g., at or above 50° C., the second polymeric layer 12with the higher coefficient of thermal expansion will expand to agreater degree than the first polymeric layer 11 having the lowercoefficient of thermal expansion. The greater expansion of the layer 12causes the label to curl toward layer 11. The deformation or curl of thelabel is reversible and the label will revert to its original shape atroom temperature. The degree of curl is a function of the filmthickness, Youngs modulus and coefficient of linear thermal expansion ofthe polymeric layers.

FIG. 1B shows the label of FIG. 1A bonded to the curved surface of acylindrical container 19. In this embodiment, the main curl direction ofthe label extends in the circumferential direction of the container. Asthe container is exposed to heat, such as the hot washing liquid used inthe bottling industry, the label curls toward the upper layer 11. As thelabel curls, the adhesive is exposed to a greater extent to the hotwashing liquid and will eventually be completely removed from thecontainer. The label of the present invention may also be applied toarticles or containers that are flat, rather than cylindrical in shape.

The coefficient of thermal expansion is determined by the equation:

Coefficient of Linear Thermal Expansion (%)=[(B−A)/A]×100  (1)

where A and B represent the measured length (cm) of a specimen of aresin after standing at 0° C. and 50° C., respectively, for 2 minutes,the specimen being 1 cm in width, 4.5 cm in length, and not more than0.5 cm in thickness as prepared at room temperature.

When subjected to the higher temperature, such as that of the washliquid, the label will curl and be removed from the underlying substrateto which it is adhered provided the curl force of the label (F_(Label))is greater than the peel adhesion of the label to the underlyingsubstrate (F_(PA)):

F_(Label)>F_(PA)

The curl force of the label, F_(Label), can be determined using thefollowing equation:

F _(Label)=(α₂−α₁)(T−T ₀)(⅛t)(E ₁ +E ₂/(E ₁ +E ₂))  (2)

wherein α₂ is the coefficient of linear thermal expansion of the bottomlayer (polymeric layer 12); α₁ is the coefficient of linear thermalexpansion of the top layer (polymeric layer 11); T is the temperature ofthe washing liquid, T₀ is the temperature at which the label is made,typically, room temperature (23° C.); t is the total thickness of thelabel; E₁ is the modulus of elasticity of the top layer (polymeric layer11) and E₂ is the modulus of elasticity of the bottom layer (polymericlayer 12).

In one embodiment, the difference between the coefficient of linearthermal expansion of the bottom layer and that of the top layer,(α₂−α₁), is greater than or equal to about 3×10⁻⁵ (1/° C.). In oneembodiment, α₂−α₁ is about 7×10⁻⁵(1/° C.).

The polymeric layers useful in the present invention do not exhibit heatshrinkage beyond the typically accepted limits of heat stable pressuresensitive adhesive films, e.g., <1% at 70° C. and <2% at 100° C. In oneembodiment, the polymeric layer 11 has a heat shrinkage of less than 4%at 80° C.

The polymeric layers 11 and 12 may comprise any of the polymeric filmmaterials described above. Each of the polymeric layers 11 and 12 may bea monolayer film or a multilayer film. In one embodiment, polymericlayers 11 and 12 are coextruded. For example polymeric layers 11 and 12may comprise a polypropylene/ethylene vinyl acetate coextrudate; apolyacrylate/polyethylene coextrudate; or a polyacrylate/ethylene vinylalcohol coextrudate. Many other coextruded combinations are possible,including coextrudates comprising more than two layers.

In one embodiment, polymeric layer 11 and polymeric layer 12 arelaminated together. Polymeric layer 12 may comprise a coextruded film ormay comprise a monolayer film. Polymeric layer 11 may comprise acoextruded film or may comprise a monolayer film. Polymeric layers 11and 12 typically have a different coefficient of linear thermalexpansion in the machine direction (MD) and have a very low coefficientof linear thermal expansion in the cross direction (CD) when the curl isin the machine direction of the label. Alternatively, when the curl isin the cross direction of the label, the polymeric layers 11 and 12 havedifferent coefficient of linear thermal expansion in the CD and have avery low coefficient of linear thermal expansion in the MD. Thedifference in the coefficient of thermal expansion can be obtained byusing different polymeric materials, for example polyethylene forpolymeric layer 12 and polyethylene terephthalate for polymeric layer11. Alternatively, the difference in coefficient of thermal expansioncan be obtained by differences in molecular orientation. For example, across direction (trans direction) oriented film for polymer layer 11 anda machine direction oriented film for polymer layer 12.

Polymeric layers 11 and 12 may be coextruded films. Alternatively, asshown in FIG. 2, the label 20 may comprise a laminated structure whereinpolymeric layer 11 is bonded to polymeric layer 12 by laminationadhesive 18. In one embodiment, polymeric layers 11 and 12 are heatsealed together.

The adhesive labels of the invention may, and generally do contain otherlayers. For example, as shown in FIG. 3, the label 30 may contain ametal layer 13 that overlies and is in contact with first polymericlayer 11. Alternatively, a print layer 14 can be on the upper surface ofpolymeric layer 11 as illustrated in FIG. 4.

In one embodiment, one of polymeric layers of the label comprises apolymeric ink layer. For example, the first polymeric layer 11 maycomprise a crosslinked ink that has been screen printed onto the secondpolymeric layer 12. Alternatively, the second polymeric layer 12 maycomprise an ink layer that has been printed onto the first polymericlayer 11.

FIG. 5 illustrates label 50 which comprises first polymeric layer 11,second polymeric layer 12 underlying first polymeric layer 11, adhesivelayer 16 underlying second polymeric layer 12, print layer 14 overlyingfirst polymeric layer 11 and transparent protective layer 15 whichoverlies and is in contact with the upper surface of the print layer 14.

FIG. 6 illustrates label 60 which is similar to the label of FIG. 5,except that label 60 contains an additional antistatic polymer layer 17between the print layer 14 and the first polymeric layer 11.

The labels of the invention may also contain adhesion promoting layersbetween one or more of the layers shown. For example, an adhesionpromoting layer can be inserted between the second polymeric layer 12and the adhesive layer 16; between the first polymeric layer 11 and themetal layer 13 or the print layer 14; etc.

The label may also contain a metal layer. As illustrated in FIG. 7,label 70 comprises first polymeric layer 11, second polymeric layer 12underlying first polymeric layer 11 and adhesive layer 16 underlyingsecond polymeric layer 12. Metal layer 13 overlies first polymeric layer11 and print layer 14 overlies metal layer 13. In FIG. 8, label 80 issimilar to the label of FIG. 7 with the addition of transparentprotective layer 15 overlying and in contact with print layer 14.

In another embodiment, illustrated in FIG. 9, the label 90 comprises afirst polymeric layer 11 having an upper surface and a lower surface, aprint layer 14 on the lower surface of the first polymeric layer 11. Thefirst polymeric layer 11 with print layer 14 thereon is bonded to thesecond polymeric layer 12 by a lamination adhesive 18. An adhesive layer16 is adhered to the lower surface of the second polymeric layer 12.

In one embodiment, the adhesive is applied to a shrink film facestock.Such films are stretched in at least one direction and, upon exposure toheat, shrink back. The shrinking of the film is irreversible. That is,upon cooling, the film does not expand back to its pre-heateddimensions. A PSA coated shrink label may be removed from the containeror substrate to which it is applied by exposing the label to heat, forexample the high temperature washing fluid, so that the label shrinksand overcomes the retention force of the adhesive label. The degree ofshrinkage of the facestock film at temperatures greater than 50° C., or60° C. or 70° C. is greater than about 5% in one embodiment, or greaterthan about 10%, or about 15 to 20% or more.

In one embodiment, the PSA label is attached to the circumferentialsurface of a cylindrical container, wherein the shrinkage label film isstretched in at least the circumferential direction. Examples ofshrinkage films include, but are not limited to, monoaxially orbiaxially stretched shrinkable polyvinyl chloride, polyester,polyethylene terephthalate, polyolefin, polyethylene, polypropylene,acetate, cocyloolefin copolymer films and combinations of two or more ofthese.

EXAMPLES

The following examples illustrate several non-limiting embodiments ofthe invention, its preparation, and performance.

Examples A-D Preparation of Base Polymers

Base polymers are prepared by free-radical emulsion polymerizationaccording to the following procedure, using the components identified inTable 2.

TABLE 2 Polymer A Polymer B Polymer C Polymer D Component mass (g) Stagede-ionized water 417.10 376.40 373.92 384.12 initial reactor potassiumpersulfate 4.76 4.55 4.74 0 loading ammonium 0 0 0 4.69 persulfatede-ionized water 198.56 220.28 223.85 226.61 pre-emulsion Disponil FES77 69.78 70.73 69.42 68.75 Disponil SUS IC 875 13.08 13.14 12.96 12.84citric acid 0 0 30.16 31.47 monohydrate 2-ethylhexyl acrylate 1312.351323.70 1306.23 1293.52 methyl methacrylate 194.43 295.56 193.94 192.05methyl acrylate 97.10 0 96.97 96.03 acrylic acid 16.30 16.42 16.20 16.05EGDMA 0.57 0 0 0 DAM 0 6.57 2.42 2.39 n-dodecyl 0.28 0 0 0 mercaptande-ionized water 116.96 118.85 116.41 119.23 initiator solutionpotassium persulfate 4.76 4.55 4.74 0 ammonium 0 0 0 4.69 persulfateActicide LA 1.17 2.52 2.42 2.39 biocide de-ionized water 52.80 46.7445.62 45.17 rinsing TOTAL 2500.00 2500.00 2500.00 2500.00

A reactor equipped with a stirrer, heat source and nitrogen inlet ispurged with nitrogen and charged with water, then heated to 78° C.Potassium persulfate or ammonium persulfate is added to the reactor, andthe reactor contents are mixed for five minutes. Thereafter, thepre-emulsion mixture is fed into the reactor over a 230 minute period.The temperature of the reactor batch is allowed to rise to 79-81° C. andthen maintained at that temperature throughout the polymerizationreaction. An initiator solution feed is commenced after the pre-emulsionfeed begins. Twenty-five minutes after the pre-emulsion feed iscomplete, the initiator feed is complete and the reactor contents areheld at 79-81° C. for 60 minutes. The reactor contents are then allowedto cool, and when the temperature reaches 38° C., a biocide is added.The rinse water is used throughout the process to wash out the variousfeed vessels and lines, and to rinse the reactor at the end of thepolymerization. Base Polymer A has a solids content of about 66% and apH of 1.8.

Examples 1-8 Preparation of Pressure Sensitive Adhesive

Using the Base Polymers described above, a series of pressure sensitiveadhesive compositions are prepared by compounding the base polymer andneutralizing it with a hydroxy functional amine or blend thereof. Inseveral examples, a salt from an organic acid is used.

Table 3 identifies the neutralizing agent, and the salt, if any, used toproduce the adhesive compositions of Examples 1-8 and ComparativeExample A. Examples 1-7 are prepared using Base Polymer A. Example 8 isprepared using Base Polymer B. Comparative Example A is Base Polymer Aneutralized with ammonia. The amount of neutralizing agent and saltlisted is in parts by weight per 100 parts dry polymer.

The abbreviations used below are as follows:

-   Bis Tris Methane 2-[bis-(2-hydroxyethyl)-amino]-2-hydroxymethyl    propane-1,3-diol-   TEA triethanolamine-   Trizma 2-amino-2-(hydroxymethyl)-1,3-propanediol-   NMDG N-methyl-D-glucamine-   H EEDA N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine-   HPEDA N,N,N′,N′-tetrakis-(2-hydroxypropyl)-ethylenediamine-   Na-Cit sodium citrate dihydrate-   Na-Glu sodium gluconate-   ammonia 12% ammonia

TABLE 3 Example Neutralizing Agent(s) Salt 1 min. 3 min. 5 min. 1 BisTris Methane 3.24 Na-Cit 2.98 25 35 45 washed off — — 2 Bis Tris Methane1.52 Na-Cit 2.98 washed off — — TEA 3.30 washed off — — 3 Trizma 1.52 —— 10 15 25 45 washed off — 4 Trizma 2.28 — — 40 45 washed off 15 washedoff — 5 Trizma 1.52 Na-Cit 2.98  7 25 40 45 washed off — 6 NMDG 2.02 — — 2  5 10 40 washed off — 7 NMDG 2.53 Na-Glu 1.49 30 45 washed off washedoff — — 8 NMDG 1.59 — — washed off — — washed off — — Comp. A Ammonia0.70 — —  2  4  6  2  4  6

Each of the adhesives of Examples 1-8 and Comparative Example A iscoated onto a siliconized polyester liner at a coat weight of 20 gramsper square meter (g/m²), and then transferred onto a curling film of thetype shown in FIG. 1A. Labels measuring 50 mm×50 mm are adhered to glassbottles (from Supplier I) of 63 mm diameter and allowed to sit for atleast 1 day at room temperature prior to testing. The curvature of thelabel, when heated, is opposite the curve of the glass bottle.

The wash-off test consists of immersing the labeled glass bottle in a 2%sodium hydroxide solution at 80° C. The label was examined after 1, 3and 5 minute intervals. If the label is not washed off, the amount oflift in mm (total of both sides of the label) is measured. A value of 50mm would mean that the label has been washed off. Each adhesiveformulation was tested twice. The labels employing the adhesives havinghydroxy functional amine neutralized copolymers have improvedremovability in hot caustic water, yet the adhesive strength of theadhesive, i.e., the peel strength, is not adversely affected.

Examples 9-12 Preparation of Pressure Sensitive Adhesive

Using Base Polymer A, pressure sensitive adhesive compositions areprepared substantially in accordance with the procedure of Example 1with the exception that varying amounts of a carbodiimide crosslinker(Carbodilite E-02 from Nisshinbo Industries) is used.

Each of the adhesives of Examples 1 and 9-12 is coated onto asiliconized polyester liner at a coat weight of 15 grams per squaremeter (g/m²), and then transferred onto a curling film of the type shownin FIG. 1A. Labels measuring 50 mm×50 mm are adhered to glass bottles(from Supplier II) of 58 mm diameter and allowed to sit for at least 1day at room temperature (RT) and for 1 day at 50° C. prior to testing.The curvature of the label, when heated, is opposite the curve of theglass bottle. Table 4 shows the results of the wash-off tests and the900 peel tests performed in accordance with FINAT FTM 2.

TABLE 4 Wt % active 90° Peel Wash-Off 90° Peel Wash-Off crosslinker on24 h at RT 24 h at RT 24 h at 50° C. 24 h at 50° C. Example dry adhesive[N/25 mm] [5 × 5 cm] [N/25 mm] [5 × 5 cm] 1 0%  6.4 cl* 33/15 8.1 cl140/160 9 1% 6.1 cl 16/9  7.3 cl 39/30 10 2% 4.7 cl 12/9  6.4 cl 58/3711 3% 3.7 cl 9/8 5.8 cl  29/180 12 4% 2.7 cl 7/7 4.3 cl 34/14 *cl =clean peel, no adhesive transfer or splitting

The wash-off test consists of immersing the labeled glass bottle in a 2%sodium hydroxide solution at 80° C. The time in seconds for the label tocompletely wash off of the bottle is measured. Each adhesive formulationwas tested twice. The addition of an external crosslinker improveswash-off, particularly on aged samples (1 day at 50° C.).

Examples 13-15 Preparation of Pressure Sensitive Adhesives

Example 13 is prepared substantially in accordance with the procedure ofExample 1, with the exception that Base Polymer C is used. Examples 14and 15 are prepared substantially in accordance with the procedure ofExample 1, with the exception that Base Polymer D is used. Base PolymersC and D both include an organic acid in the polymerization pre-emulsion.

Each of the adhesives of Examples 13-15 is coated onto a siliconizedpolyester liner at a coat weight of 20 grams per square meter (g/m²),and then transferred onto a curling film of the type shown in FIG. 1A.Labels measuring 50 mm×50 mm are adhered to glass bottles (from SupplierII) of 58 mm diameter and allowed to sit for at least 1 day at roomtemperature (RT) and for 3 days at 50° C. prior to testing. Thecurvature of the label, when heated, is opposite the curve of the glassbottle. Table 5 indicates the neutralizing agent(s) used and shows theresults of the wash-off tests and the 900 peel tests. The amount ofneutralizing agent listed is in parts by weight per 100 parts drypolymer.

TABLE 5 90° C. Peel 90° C. Peel Wash Off Neutralizing 24 h at RT >72 hat 50° C. >72 h Example Agent(s) [N/25 mm] [N/25 mm] at 50° C. 13 BisTris 2.94 3.5 cl 6.1 cl 40/45 Methane TEA 3.30 14 Bis Tris 2.94 3.8 cl10.4 cl  15/35 Methane TEA 3.30 15 HEEDA 3.20 3.9 cl 9.2 cl 35/45 TEA2.89

The wash-off test consists of immersing the labeled glass bottle in a 2%sodium hydroxide solution at 80° C. The time in seconds for the label tocompletely wash off of the bottle is measured. Each adhesive formulationwas tested twice. The wash off times for the labels remains low afterageing and the peel strength is more stable after ageing than adhesiveswherein no organic acid is added to the pre-emulsion and no externalcrosslinker is used.

While the invention has been explained in relation to embodiments, it isto be understood that various modifications thereof will become apparentto those skilled in the art upon reading the specification. Therefore,it is to be understood that the inventions disclosed herein is intendedto cover such modifications as fall within the scope of the appendedclaims, and to cover insubstantial variations thereof.

1. An emulsion based pressure sensitive adhesive composition comprising;an acrylic pressure sensitive adhesive copolymer of a plurality ofcopolymerizable monomers comprising at least one ethylenicallyunsaturated carboxylic acid; wherein the composition is neutralized to apH of greater than 4.5 with at least one hydroxy functional amine; andwherein the composition is removable upon exposure to an aqueous causticsolution at a temperature of at least 50° C.
 2. The composition of claim1 wherein the copolymer comprises a copolymer of (a) about 75% to about90% by weight of one or more alkyl acrylates having from about 4 toabout 12 carbon atoms in the alkyl group; (b) about 0.25% to about 5% byweight of one or more acid monomers; (c) about 10% to about 20% byweight of at least one hard monomer; (d) 0 to about 1% by weight of atleast one multifunctional crosslinking monomers.
 3. The composition ofclaim 2 wherein the one or more alkyl acrylates is selected from butylacrylate, hexyl acrylate, heptyl acrylate, n-octyl acrylate, iso-octylacrylate, isobutyl acrylate, 2-ethylhexyl acrylate, and nonyl acrylate,and mixtures of two or more thereof.
 4. The composition of claim 2wherein the one or more acid monomers is selected from acrylic acid,methacrylic acid, itaconic acid, beta-carboxyethyl-acrylate and mixturesof two or more thereof.
 5. The composition of claim 2 wherein the one ormore hard monomers is selected from methyl and ethyl acrylate, methyland ethyl methacrylate, vinyl acetate, styrene and mixtures of two ormore thereof.
 6. The composition of claim 2 wherein the multifunctionalmonomer is selected from diallyl maleate, ethylene glycoldimethacrylate, tripropylene glycol diacrylate, allyl methacrylate,divinyl benzene, N,N′-methylene-bis-acrylamide, triallyl cyanurate, andmixtures of two or more thereof.
 7. The composition of claim 1 whereinthe hydroxy functional amine is selected from triethanolamine,2-[bis-(2-hydroxyethyl)-amino]-2-hydroxymethyl propane-1,3-diol,2-amino-2-(hydroxymethyl)-1,3-propanediol, N-methyl-D-glucamine,N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine,N,N,N′,N′tetrakis-(2-hydroxypropyl)-ethylenediamine and mixtures of twoor more thereof.
 8. The composition of claim 1 wherein the hydroxyfunctional amine comprises2-[bis-(2-hydroxyethyl)-amino]-2-hydroxymethyl propane-1,3-diol.
 9. Thecomposition of claim 1 further comprising an external crosslinkingagent.
 10. The composition of claim 1 further comprising an organic acidneutralized by the hydroxy functional amine.
 11. A facestock materialcomprising a coating of the composition of claim
 1. 12. The facestockmaterial of claim 11 wherein the facestock comprises (a) a firstpolymeric layer having a first coefficient of thermal expansion; and (b)a second polymeric layer having a second coefficient of thermalexpansion underlying the first polymeric layer, wherein the firstcoefficient of thermal expansion is less than the second coefficient ofthermal expansion.
 13. A removable label comprising: a first polymericlayer having a first coefficient of thermal expansion; a secondpolymeric layer having a second coefficient of linear thermal expansionunderlying the first polymeric layer, wherein the first coefficient oflinear thermal expansion is less than the second coefficient of linearthermal expansion; and an adhesive layer underlying the second polymericlayer, the adhesive layer comprising an emulsion based pressuresensitive adhesive composition comprising an acrylic pressure sensitiveadhesive copolymer of a plurality of copolymerizable monomers comprisingat least one ethylenically unsaturated carboxylic acid; wherein thecomposition is neutralized to a pH of greater than 4.5 with at least onehydroxy functional amine; and wherein the label is reversibly curledtoward the first polymeric layer at a temperature at or above 50° C. andthe adhesive is removable upon exposure to an aqueous caustic solutionat a temperature of at least 50° C.
 14. The label of claim 13 whereinthe label is bondable to a cylindrically curved surface of an articleand the main expansion direction of the label extends in thecircumferential direction of the article.
 15. The label of claim 13wherein the first polymeric layer and the second polymeric layer arecoextruded.
 16. The label of claim 13 wherein the first polymeric layerand the second polymeric layer are bonded to each other by a laminationadhesive.
 17. The label of claim 13 further comprising a metal layer.18. The label of claim 17 wherein the metal layer overlies the firstpolymeric layer.
 19. The label of claim 13 further comprising a printlayer.
 20. The label of claim 19 wherein the print layer overlies thefirst polymeric layer.
 21. The label of claim 13 wherein the pressuresensitive adhesive copolymer comprises a copolymer of (a) about 75% toabout 90% by weight of one or more alkyl acrylates having from about 4to about 12 carbon atoms in the alkyl group; (b) about 0.25% to about 5%by weight of one or more acid monomers; (c) about 10% to about 20% byweight of at least one hard monomer, (d) 0 to about 1% by weight of atleast one multifunctional crosslinking monomers.
 22. The label of claim13 wherein the hydroxy functional amine is selected fromtriethanolamine, 2-[bis-(2-hydroxyethyl)-amino]-2-hydroxymethylpropane-1,3-diol. 2-amino-2-(hydroxymethyl)-1,3-propanediol,N-methyl-D-glucamine,N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine,N,N,N′,N′-tetrakis(2-hydroxypropyl)-ethylenediamine and mixtures of twoor more thereof.
 23. The label of claim 19 wherein the print layerunderlies the first polymeric layer.
 24. A removable label comprising: apolymeric layer comprising at least one shrinkable film wherein the filmis stretched in at least one direction; an adhesive layer underlying thepolymeric layer, the adhesive layer comprising an emulsion basedpressure sensitive adhesive composition comprising an acrylic pressuresensitive adhesive copolymer of a plurality of copolymerizable monomerscomprising at least one ethylenically unsaturated carboxylic acid;wherein the composition is neutralized to a pH of greater than 4.5 withat least one hydroxy functional amine; and wherein the polymeric layerirreversibly shrinks at a temperature at or above 50° C. and theadhesive is removable upon exposure to an aqueous caustic solution at atemperature of at least 50° C.
 25. A method of detaching a label from anarticle, wherein the label comprises: a first polymeric layer having afirst coefficient of linear thermal expansion; a second polymeric layerhaving a second coefficient of linear thermal expansion underlying thefirst polymeric layer, wherein the first coefficient of linear thermalexpansion is less than the second coefficient of linear thermalexpansion; and an adhesive layer underlying the second polymeric layer,the adhesive comprising an emulsion based pressure sensitive adhesivecomposition comprising an acrylic pressure sensitive adhesive copolymerof a plurality of copolymerizable monomers comprising at least oneethylenically unsaturated carboxylic acid; wherein the composition isneutralized to a pH of greater than 4.5 with at least one hydroxyfunctional amine; the method comprising heating a washing liquid to forma hot washing liquid; and gradually detaching the label from thearticle, wherein the step of gradually detaching the label from thearticle comprises the step of exposing the label to the hot washingliquid for at least a predetermined amount of time so that the label isreversibly curled toward the first polymeric layer thereby overcomingthe adhesive force of the adhesive layer.
 26. The method of claim 25wherein the liquid comprises caustic soda.
 27. The method of claim 25wherein the hot washing liquid has a temperature in the range of about50° C. to about 90° C.
 28. The method of claim 25 wherein the pressuresensitive adhesive copolymer comprises a copolymer of (a) about 75% toabout 90% by weight of one or more alkyl acrylates having from about 4to about 12 carbon atoms in the alkyl group; (b) about 0.25% to about 5%by weight of one or more acid monomers; (c) about 10% to about 20% byweight of at least one hard monomer; (d) 0 to about 1% by weight of atleast one multifunctional crosslinking monomers.